Billy Fitzgibbons is the young son of Farmer Fitzgibbons and Mrs. Fitzgibbons. He's a bit of a mischievous boy, who enjoys catching small animals and keeping them as pets (which he tried on Mrs. Brisby when she was in the Fitzgibbons' kitchen, drugging Dragon's food).
Quote: "Mom, don't let the cat in yet, I've caught a mouse!"
Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Billy Fitzgibbon (the family's name has no "s" on the end in the book) is the younger of two sons of Farmer and Mrs. Fitzgibbon. The older son, Paul, is described this way: "Paul, at fifteen, was a quiet, hardworking boy, rather clumsy in his movements but strong and careful about his chores." He even expresses concern for Mrs. Frisby after Billy catches her in the kitchen. However, we don't ever see Paul in the movie. We do, however, briefly see Billy, "who, at age twelve was noisier and had an annoying habit of skimming rocks across the grass at anything that moved. Mrs. Frisby did not care much for Billy."
In the animated sequel: Though we do see the Fitzgibbons farm in the sequel, none of the Fitzgibbons family itself are ever seen during it.